Replace zero with Null (2003)

In a query, I have a field called FA Members. Whenever there is a zero, I would like it to be Null. How would I define a new column, so that I have that field with the 0's defined as null. FAMem2: IIF([FAMembers] = 0,?,[FAMembers]). I wasn't sure how to define null and of course my syntax is probably wrong.